“Den” refers to something very specific in snake biology: the hibernaculum, a sheltered underground space where snakes overwinter, often communally and often for generations. Rock crevices, old animal burrows, and cave systems all make good dens, offering stable, frost-free temperatures that snakes can’t access by staying out in the open once the weather turns cold.
What makes snake dens especially interesting is site fidelity. Many snake species return to the exact same den year after year, sometimes traveling considerable distances to do it, and a single well-established den can be used by the same population for decades. Multiple species can even share a den at once, since the shelter itself, not social bonding, is what draws them together.
Once temperatures warm in spring, the den empties out fast as its occupants disperse to hunt, mate, and spend the summer largely alone, until the cycle brings them back again next winter.
Example: “Herpetologists have tracked the same den of snakes returning to this rock outcrop for over a decade.”
Related: Collective Nouns for Reptiles · What Is a Group of Snakes Called?